The present application relates to preconditioning lithium niobate and lithium tantalate crystals.
Lithium niobate (LiNbO3, “LN”) and lithium tantalate (LiTaO3, “LT”) single crystals show a variety of interesting and useful physical properties. At room temperatures, LN and LT crystals have a ferroelectric order that is a spontaneous electric polarization. The LN and LT crystals also show strong electro-optic coupling, pressure-electricity coupling called piezoelectric effect, and temperature-electricity coupling called pyroelectric effect.
LN and LT crystals are used in many electronic, optical, and electro-optical devices, such as surface acoustic wave filters or other frequency filters, optical modulators, electro-optical switches, or detectors using piezoelectric or pyroelectric effects. While a particular property of an LN or LT crystal may be useful for one application, the same property can have undesired effects in another application or under certain conditions. For example, a pressure sensor and a heat detector take advantage of the piezoelectric and pyroelectric effects, respectively. However, if the crystal is subject to excessive mechanical stress or temperature changes during manufacturing or operation, the piezoelectric and pyroelectric effects can build up electric charges on the surface of the crystal. The built up charges decay slowly, typically in the order of several hours, and can interfere with the operation of the device or even damage the device by triggering electric sparks. Such undesired effects can be minimized by preconditioning the crystals.
In one preconditioning technique, a LN or LT crystal is chemically reduced by heating the crystal in a reducing atmosphere. On the surface of the crystal, electric charges decay at a higher rate after the reduction than without preconditioning. To accelerate the charge decay, the crystal's reduction can be increased by increasing the temperature or the duration of the heating during preconditioning. If the decay rate is high enough, surface charges cannot build up to a level that would damage the operation of a device including the reduced crystal.